This invention relates to an all-solid-state electrochromic display device and an organic electrochromic display device. A display device is often referred to either as a display panel or merely as a display.
As will later be described in detail with reference to two of eight figures of the accompanying drawings, a typical electrochromic display device comprises a base electrode, an electrolyte layer or film on the base electrode, an electrochromic layer or film on the electrolyte layer, and a display electrode on the electrochromic layer. The base electrode will herein be referred to, in relation to the display electrode, as a counter electrode. The display electrode is substantially transparent. The counter electrode may or may not be transparent. The display electrode may consist of a plurality of segmented electrodes arranged in a predetermined geometrical configuration. Alternatively, the display and the counter electrodes may provide a matrix of electrodes. The electrochromic layer is a layer or film of an electrochromic material, which will become clear as the description proceeds. As seen from the foregoing, an electrochromic display device comprises a substantially transparent electrode, a counter electrode, and an electrochromic layer between the transparent and the counter electrodes.
When a signal or electric voltage is applied to the display electrode as an operating voltage relative to the counter electrode, the electrochromic material is readily subjected to a redox reaction, namely, either reduced or oxidized. The redox reaction of the electrochromic material results in a reversible variation in the absorption spectrum in the visible range. The electrochromic display device therefore displays a visible display. Such an electrochromic display device is promising because the visible display is distinctly colored, scarcely dependent on the angle of viewing, and clear even when exposed to direct rays.
Various inorganic and organic electrochromic materials are already known. Examples are oxides of transition metals, such as tungsten oxide, aromatic or heterocyclic compounds, and organometallic compounds, namely, coordination compounds or complexes of transition metals and aromatic or heterocyclic compounds. It should be noted that various organometallic compounds are organic compounds in general and that the expression an "organic" electrochromic material may or may not mean an "organometallic" electrochromic material.
In contrast to the fact that the color displayed by an inorganic electrochromic material is restricted to deep blue, organic electrochromic materials are capable of displaying a number of colors when various functional groups or radicals are substituted for terminal or end groups of the aromatic or heterocyclic compounds.
The electrolyte layer has mostly been manufactured of a liquid electrolyte. An electrochromic display device of this type is defective. The device must comprise a hermetic casing for the liquid. The structure is therefore intricate. The liquid electrolyte must be forced into the casing. This complicates the process of manufacture. Even with the intricate structure and the complicated process, the electrolyte can leak out of the casing due, for example, to thermal expansion and can thereby damage other electronic devices.
It is known to use insulating materials, such as silicon monoxide, calcium fluoride, magnesium fluoride, and chromium sesquioxide, as solid electrolytes. In an all-solid-state electrochromic display device including an insulator solid electrolyte layer, the redox reaction takes place in the electrochromic layer as a result of the action of moisture absorbed in the insulating material. The redox reaction is therefore seriously influenced by surrounding conditions, above all, by humidity. Furthermore, the absorbed moisture gives rise to bubbles on occurrence of the redox reaction. This adversely affects the reliability of operation of the electrochromic display device.
Other known solid electrolytes are ion conductive materials as, for example, lithium nitride and beta alumina in which the lithium or sodium ions contribute to the ionic conduction. An all-solid-state electrochromic display device including a lithium or sodium ion conductive solid electrolyte layer is disadvantageous because the response is slow due to the small ion mobility. Moreover, such an electrolyte layer does not tenaciously adhere to the electrochromic layer and/or the counter electrode. Chemical reaction tends to occur at the interface. As a result, the life is short.
At any rate, such a solid electrolyte layer has been used in combination with inorganic electrochromic materials. A typical all-solid-state electrochromic display device of this type has a response speed of ten seconds for a display of a contrast of 3:1 when the display is exposed to rays of the daylight color or to a white color which are either incident onto the display electrode or onto both the display electrode and the transparent counter electrode. Such a contrast will be called a daylight contrast.
The organic electrochromic materials have been used in combination with liquid electrolytes. The electrochromic display devices of this type are described, for example, in an article contributed by C. J. Schoot et al to Applied Physics, Letters, Volume 23, No. 2 (July 15, 1973), pages 64-65, under the title of "New Electrochromic Memory Display," and in another article contributed by L. G. Van Uitert et al to Applied Physics, Letters, Volume 36, No. 1 (Jan. 1, 1980), pages 109-111, under the title of "Anthraquinone Red Display Cells." All-solid-state electrochromic display devices have not yet been known, in which an organic electrochromic layer is used. A electrochromic display device comprising an organic electrochromic layer is herein called an organic electrochromic display device.